Gallach Sanchez, David

Kortenhaus, Andreas

Troch, Peter

English

Gallach-Sánchez, D.

Troch, P.

Kortenhaus, A.

Open Marine Archive

WAVE OVERTOPPING ON STEEP LOW-CRESTED STRUCTURES:
ANOTHER CLIMATE CHANGE CHALLENGE
DEPARTMENT OF CIVIL ENGINEERING
David Gallach-Sánchez, Peter Troch, Andreas Kortenhaus
We need a good understanding of wave overtopping
to protect the Belgian coastline.
Climate change is responsible for sea level rise and increase in the storminess (i.e. more frequent
and more severe storms), posing risks to coastal communities. 
In this climate change scenario, the existing coastal structures protecting the Belgian coastline
against wave attack should remain at least equally effective against future storms.
Wave overtopping is the coastal process occurring when a volume of water passes over
a sea defence structure when waves attack the structure. It is a key design parameter as
its tolerable value determines the necessary crest level of this type of structures (e.g. breakwaters).
As sea levels rise, the crest freeboard of sea defence structures will decrease leading to
more wave overtopping. A good knowledge of the physics behind wave overtopping
is then necessary to improve the safety of this type of structures.
To improve the knowledge of wave overtopping ,
we performed physical model tests in a wave flume.
Although wave overtopping has been widely investigated in literature,
there is still a knowledge gap on overtopping for very steep slopes and 
vertical walls with very small and zero freeboards (steep low-crested structures).
To cover this knowledge gap, we performed wave overtopping tests in the wave flume of
Ghent University on smooth and impermeable steep low-crested structures, obtaining
two new datasets called UG13 (overtopping in deep water conditions) and UG14
(overtopping in shallow water conditions).
david.gallachsanchez@ugent.be
awww.ugent.be
Wave overtopping damaging the railway infrastructure in Dawlish (UK).
Wave flume at Ghent University. Wave overtopping during a test.
Cross section of the test set-up with indication of overtopping parameters.
The average overtopping rate depends on
the relative freeboard and the slope angle.
Dataset UG13 is formed by overtopping data for very steep slopes and
vertical walls, with very small and zero freeboard, in deep water conditions.
The overtopping rate decreases for increasing values of relative freeboard,
while it increases for milder slopes. 
The data for shallow water conditions show an increase
of overtopping for larger relative freeboards.
Dataset UG14 is an extension of dataset UG13 with overtopping data for  
steep low-crested structures in shallow water conditions.
The overtopping behaviour in shallow water is similar as in deep water,
however the overtopping rate increases for larger relative freeboards.
Source: APEX ©/ The Telegraph
Average wave overtopping rate as a function of the relative freeboard for dataset UG13
Troch et al. (2015). Experimental study of overtopping performance for the cases of very steep slopes and vertical 
walls with very small freeboards. Proceedings of the 34th International Conference on Coastal Engineering 
Average wave overtopping rate as a function of the relative freeboard for dataset UG14
Gallach-Sánchez et al. (2014). Experimental study of overtopping performance of steep smooth slopes
for shallow water wave conditions. Proceedings of Coastlab14 


Wave overtopping on steep low-crested structures: another climate change challenge

- 35 - 
 
 
Wave overtopping on steep low-crested structures: another climate 
change challenge 
Gallach-Sánchez David
1
, Troch Peter
1
 and Kortenhaus Andreas
1 
1 
Department of Civil Engineering, Ghent University, Technologiepark 904, 9052 Zwijnaarde (Ghent), 
Belgium 
E-mail: David.GallachSanchez@UGent.be  
 
Climate change is responsible for sea level rise and increase in the storminess (i.e. more frequent and 
more severe storms), posing risks to coastal communities. Floods on the coastline are expected to 
increase, leading to potential human life losses and significant economic damage. In this climate 
change scenario, the existing coastal structures protecting the Belgian coastline against wave attack 
should remain at least equally effective against future storms. Therefore, good knowledge of the 
coastal processes is required to correctly assess the safety of the existing coastal structures in this new 
scenario and to improve design guidelines. The main coastal processes involved are, among others, 
wave overtopping over the crest of the structures, wave run-up on sea dikes and wave forces and 
pressures exerted by the waves attacking the structures. 
 
Wave overtopping is a key design parameter as it determines the necessary crest level of coastal 
structures (e.g. breakwaters) that limits the amount of water passing over the structure during wave 
attack. Traditional research has focused on analysing the average wave overtopping rate establishing 
its relation with diverse wave parameters of the sea state (wave height, wave period, wave steepness, 
etc.) and structural parameters (slope angle, crest freeboard, etc.). Furthermore, physical insights show 
that the damage on infrastructures and people near the coast during a storm is also related to 
individual volumes from single wave overtopping events. 
 
Despite the large scientific literature available, there are still knowledge gaps to be covered in order to 
improve the understanding of wave overtopping under different conditions. This knowledge gap 
consists of overtopping data for steep low-crested structures (coastal structures with steep slopes and 
vertical walls, with a crest freeboard ranging from small to zero). This type of coastal structures with 
small freeboards are relevant in a climate change scenario where the sea level is increasing while the 
crest level of the existing structures is impossible or very expensive to modify. Improving the 
knowledge on various processes related to wave overtopping will eventually lead to more accurate 
overtopping prediction formulae with larger ranges of application, and hence to safer coastal defence 
structures under a climate change scenario. 
 
The EurOtop (2007) manual is the reference manual in Europe about wave overtopping and 
overtopping assessment of coastal structures. It contains several overtopping prediction formulae for 
various types of coastal structures, lacking however prediction formulae valid for steep low-crested 
structures. An updated version, EurOtop (2016), is available with improved overtopping prediction 
formulae. 
 
To extend the wave overtopping data available in the scientific literature, we performed 2D physical 
model tests at the large wave flume of Ghent University (Belgium). The experiments consisted in 
overtopping tests on smooth impermeable coastal structures, both with deep and shallow water 
conditions. The tests were focused on obtaining average and individual wave overtopping data for a 
range of slope angles α from steep to vertical walls and for a range of relative crest freeboards R
c
/H
m0
 
(where R
c
 is the crest freeboard of the structure and H
m0 
is the significant wave height) from large to 
zero. Four different datasets resulted from these physical model tests: dataset ‘UG10’, ‘UG13’, ‘UG14’ 
and ‘UG15’, all of them focusing on steep low-crested structures. 
 
These new datasets obtained at Ghent University are useful to improve the accuracy of the existing 
prediction formulae on the range of steep low-crested structures. Recently, van der Meer & Bruce 
(2014) presented a new overtopping prediction formula fitted through the UG10 dataset and thus 
being in the range of application of steep low-crested structures. However, this formula underpredicts 
the results of the UG13, UG14 and UG15 datasets for very steep slopes and vertical walls, and for the 
zero freeboard limit case. By using the new Ghent University overtopping datasets, improvements on 
the accuracy of the prediction formulae can be made. 
 
Keywords: coastal engineering; climate change; coastal structures; wave overtopping; steep low-crested 
structures; physical modelling  
- 36 - 
 
 
 
Reference 
- EurOtop, 2016.  Manual on wave overtopping of sea defences and related structures.  An overtopping 
manual largely based on European research, but for worldwide application.  Van der Meer, J.W., 
Allsop, N.W.H., Bruce, T., De Rouck, J., Kortenhaus, A., Pullen, T., Schüttrumpf, H., Troch, P. and 
Zanuttigh, B., www.overtopping-manual.com.  
 
  


Ghent University Academic Bibliography

https://biblio.ugent.be/publication/8515887/file/8515888.pdf

Wave overtopping on steep low-crested structures: another climate change challenge

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